Lisa, I agree with all the advice about having a hot-enough flame.
However, some solders just seem to be from a bad batch (or
something). In my collection of sheet solders I use on base metal, I
have one from Rio Grande (101-251 – when I bought it, I was told
that the item was being liquidated, a good idea!). I work primarily
with base metal, occasionally soldering silver to it. After trying
the aforementioned solder a few times, I marked it, “does not flow
well” and will not use it any more.

A couple of years ago, I bought some wire solder for brass from
Metalliferous – it also is dreadful. No matter how much I heated it,
it would not melt evenly. I still have it on hand for "historical"
reasons but will never use it again. Other solders I’ve gotten from
Rio and miscellaneous other places have worked fine. If you want to
take a chance and use some of the older cadmium-containing solders
(e.g., “Easy-Flo”), they still are available at Indian Jewelers
Supply. They ordinarily flow very well, but one must take care to
have exhaust fans and to wear a mask meant to filter out metal
fumes.